This invention relates to a semiconductor device in which patterning is effected using a silicon oxynitride (SiON) based thin film as an antireflection film. More particularly, it relates to such semiconductor device prohibited from being deteriorated in electrical properties due to hydrogen contained in the SiON based thin film.
As a material for metallization of a semiconductor device, an aluminum (Al) based alloy or a high melting metal silicide is used extensively. It is becoming indispensable to provide an antireflection film on the surface of such material having high light reflectance for the purpose of improving accuracy in photolithography. The reason is that, in keeping pace with the tendency towards a finer design rule of the semiconductor device, the light exposure wavelength for a photoresist coating film shifts to a short wavelength side and, in addition, the pattern size is approaching to the wavelength of the exposure light, so that stable resolution is becoming difficult to achieve on a layer of a material exhibiting high light reflectance.
Above all, if an exposure light source having strong monochromaticity, such as excimer laser light, is used, standing wave effects are manifested intensively unless an anti-reflection film is used, with the result that a resist pattern may be deformed or the resulting metallization pattern tends to be fluctuated in linewidth.
As such antireflection film, an SiON based thin film is attracting attention because its optical constants may be set arbitrarily. Since the SiON based thin film can be formed by a chemical vapor deposition (CVD) method using a plasma, and the optical constants thereof may be controlled by changing the atomic composition ratio in the film, it can be applied to photolithography employing the excimer laser light.
FIG. 1 shows, as a semiconductor device in which an antireflection film comprised of an SiON based thin film used for photolithography is left, a static RAM (SRAM) in which connection across a thin film transistor (TFT) as a load and a memory node is achieved via a contact hole which has been opened in a self-aligned manner. With the present SRAM, a contact hole 4 is opened in a self-aligned manner in an SiO based interlayer insulating film 3 between two neighboring gate electrodes 2 on a silicon (Si) substrate 1. A polysilicon metallization layer 6 is formed while burying the contact hole 4 for electrically interconnecting the Si substrate 1 and a polysilicon metallization layer 6. Although not shown, an antireflection film used during patterning of the gate electrode 2 is left on the SRAM.
Referring to FIGS. 1 to 3, the process employing the antireflection film is explained. Referring first to FIG. 2, a gate insulating film 7 is formed by thermal oxidation on the Si substrate 1. After forming a polysilicon layer 8 and a tungsten silicide layer 9, and forming an antireflection film 10 on the tungsten silicide layer 9, an offset oxidized film 11 is formed, followed by a photoresist coating film 12. Selective light exposure of the photoresist coating film 12 is then performed while the strong light of reflection from the tungsten silicide layer 9 is prohibited by the antireflection film 10. Then, using a photoresist pattern, formed by a development process, as a mask, the offset oxidized film 11, the antireflection film 10, tungsten silicide layer 9 and the polysilicon layer 8 are etched sequentially for patterning the gate electrode 2 to a desired configuration.
Then, as shown in FIG. 1, a sidewall 13 and the SiO based insulating film 3 are formed, and the contact hole 4 is opened in a self-aligned manner between the neighboring gate electrodes 2. The polysilicon metallization layer 6 is formed while burying the contact hole 4, and the polysilicon layer 8 is patterned to produce the SRAM shown in FIG. 1.
In the course of a process subsequent to formation of the gate electrode 2, it is desirable that the reflected light from the tungsten silicide layer 9 be prohibited during the photolithographic process. The antireflection film 10 previously formed on the gate electrode 2 is useful for this purpose.
With the above-described SRAM, an antireflection film formed of a SiON based thin film is used for formation of an upper metallization layer. Specifically, an antireflection film 18 is employed when a SiO based interlayer insulating film 14 is formed for covering the polysilicon metallization layer 6, a via-hole 15 is opened in the SiO based interlayer insulating film 14, an Al metallization layer 17 is formed on the overall surface while burying the via-hole 15, and the Al metallization layer 17 is subsequently patterned.
That is, by forming the antireflection layer 18 after formation of the Al metallization layer 17 and before coating a photoresist coating film 19, the photoresist coating film 19 may be selectively exposed to light while the strong light of reflection from the Al metallization layer 17 is prohibited, as a result of which the Al metallization layer 17 may be patterned to a desired configuration. The Al metallization layer 17 and the polysilicon metallization layer 6 may be electrically connected by an Al plug 16 embedded in the via-hole 15.
When forming further metallization on the thus patterned Al metallization layer 17, an antireflection film is similarly required. That is, as shown in FIG. 5, the strong light of reflection from the Al metallization layer 17 needs to be prohibited during selective light exposure of a photoresist coating film 21 if a via-hole is to be opened in the SiO based interlayer insulating film 20 formed for covering the Al metallization layer 17. For this reason, the antireflection film 18 on the Al metallization layer 17 is left after patterning of the Al metallization layer 17 and again used during selective light exposure of the photoresist coating film
During the production process for the SRAM, as described above, the antireflection films 10, 18 formed of SiON based thin films are used for suppressing the effect of the reflected light from the underlying layer during selective light exposure of the photoresist coating films 12, 19 and 21.
With the wafer produced by the above process, SiON based thin films persist as the antireflection films 10, 18 on the gate electrode 2 and the Al metallization layer 17. These SiON based thin films contain hydrogen in an amount of the order of 20% such that, when heat is applied during the process of passivation or annealing for activating impurities subsequent to formation of the antireflection films 10, 18, the phenomenon of hydrogen diffusion to the ambient area occurs. If hydrogen diffused in this manner reaches the gate insulating film 7, there is a risk of deteriorating the so-called resistance against hot carriers.
In order to avoid this phenomenon, it may be contemplated to remove the antireflection film each time it has been employed. However, since it may occur that a proper selection ratio between the antireflection film and the underlying layer cannot be maintained or the same antireflection film can be used only once, a step of forming the antireflection film needs to be carried out for each photographic process.